In general, metal cans are roughly classified into two-piece cans and three-piece cans. Two-piece cans refer to cans formed of two parts, namely, of a can body integrally formed with a can bottom and a lid. Three-piece cans refer to cans formed of three parts, namely, a can trunk, a top lid, and a bottom lid. The can body of a two-piece can (sometimes abbreviated as “two-piece can body”) has no seam (welded part) so that it gives aesthetically pleasing appearance. However, the two-piece can'aesthetically pleasing requires a high strain level. Since the can trunk of a three-piece can has a seam, it is inferior, in appearance to the two-piece can. The three-piece can, however, generally requires only a low strain level. Therefore, there is a tendency that two-piece cans have often been used for small-sized high-quality articles and three-piece cans have often been used for large-sized low-priced articles in the market.
As a metal base material for two-piece can bodies having a high degree of drawing and having a high degree of stretching in the height direction of the cans (hereinafter sometimes simply referred to as “having a high strain level”), such as aerosol cans, among two-piece cans, an expensive thick aluminum sheet is generally used, and steel sheet raw materials, such as an inexpensive thin tinplate or tin-free steel, are rarely used. The reason is that since the two-piece aerosol can has a very high strain level, a high strain processing, such as drawing or DI, is difficult to apply to steel sheets, and in contrast, soft metallic materials, such as aluminum, can be subjected to impact shaping.
Under such circumstances, it is industrially very important to produce a can body of the two-piece can bodies having a high strain level using an inexpensive, thin, but high-strength steel sheet material, such as tinplate or tin-free steel.
In contrast, with respect to usual two-piece cans having a low strain level, techniques of producing the usual two-piece can by drawing or DI processing using resin-coated steel sheets (hereinafter referred to as “laminated steel sheets”) as a raw material have been conventionally known.
As a resin of covering a laminated steel sheet used for production of such a two-piece can body having a low strain level, polyester is generally used. In particular, ionomer compound materials containing, as a main phase, polyethylene terephthalate, an ethylene terephthalate-isophthalate copolymer, an ethylene terephthalate-butylene terephthalate copolymer, and saturated polyester are mentioned. The above-mentioned polyesters are designed according to methods of producing a two-piece can body having a low strain level, and are suitable for the application. However, no investigation has been conducted on a method of producing a can body that requires neck-in processing having a high strain level after drawing as in two-piece aerosol cans, for example.
Patent Documents 1 to 3 disclose drawing and DI processing techniques for resin-coated metal sheets. However, any techniques disclosed therein are directed toward can bodies having a low strain level, such as beverage cans and food cans. Specifically, techniques are disclosed in which, in production of two-piece cans having a low strain level, internal stress caused by processing is relieved by performing heat treatment after the processing or the orientation of resin is positively promoted.
Patent Documents 2 and 3 disclose performing heat treatment in an intermediate or final stage while aiming at preventing delamination of a resin layer or providing barrier properties after processing. More specifically, Patent Document 2 proposes heat treatment using an oriented thermoplastic resin for the purpose of relieving internal stress and promoting oriented crystallization. The heat treatment has generally been used for beverage cans. Moreover, Patent Document 2 discloses that the heat treatment is preferably conducted to a redrawn cup at or below a temperature at which a coated resin is sufficiently crystallized (at the melting point or 5° C. lower than the melting point). However, considering the description of Examples, it is revealed that only cans having a low strain level are targeted.
Moreover, Patent Document 3 discloses an example in which a resin formed of saturated polyester and an ionomer compound is provided on a coating layer, and then DI processing is performed. Patent Document 3 describes a processing method in which heat treatment is performed after drawing, and then DI processing, necking, and flanging are performed. However, considering the description of Examples, it is revealed that only cans having a low strain level are targeted similarly as in Patent Document 2.
Patent Documents 4 and 5 disclose methods for relieving internal stress by performing heat treatment on a can principally at or above the melting point of a resin after shaping the can. However, the strain level of the obtained can body is still low considering the specification and the description of Examples.
Patent Document 1: Japanese Examined Patent Application Publication No. 7-106394
Patent Document 2: Japanese Patent No. 2526725
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-148324
Patent Document 4: Japanese Examined Patent Application Publication No. 59-35344
Patent Document 5: Japanese Examined Patent Application Publication No. 61-22626
More specifically, hitherto, methods of producing high strain bodies of two-piece cans, such as aerosol cans, using a laminated steel sheet have never been disclosed.
The present inventors have attempted to produce high strain two-piece can bodies by shaping a circular laminated steel sheet into a cylindrical form having a bottom by DI processing, and then subjecting a portion around an opening thereof to neck-in processing. Then, a resin layer is delaminated and broken. These phenomena are problems peculiar to high-strain shaping, and heat treatment is apparently effective for overcoming the problems. However, heat treatment before or after shaping in conventional findings is not enough for overcoming the problems, and delamination of a resin layer has not been prevented in a high strain level area. Thus, even when prior art techniques have been simply applied to production of can bodies of high strain two-piece cans, the problem of delamination of a resin layer has not been overcome. Moreover, another problem such that processability of a resin layer is deteriorated at steps after a heat treatment step has arisen.
The present invention has been made under the above-described circumstances, and aims to provide a laminated steel sheet suitable as a raw material for two-piece aerosol cans which are free from delamination and breakage of a resin layer, a method of producing a two-piece can body, and a two-piece can body.